And the back of a 737-900 may have cool lighting and tv's, the the back of a 757-300 May really give you the "Mail tube" feel... Both could use some width and space. Sure, it's not a wide body, but the back of a 767 vs a 737/757 is a very different feel, even if the seats are the same. So... If the 73 and 75 share a fuselage (really not surprised if that's the fact, just haven't checked) in stretching the 73. Why not make things a bit wider? Yea, I know, "cost", but for passenger comfort a little more ambient space, maybe even an inch between rows and an inch wider seat, would make the long routes (up to an including crossings) a bit better. With the metallurgy of today, more space and lower weight can be achieved... That was the crux of the 787 sales pitch outside of motors.
I don't have a dog in the fight, but the idea of continually stretching a 737 instead of a 757 rebirth is something I can't grasp. Sure, there are costs in rebuilding a factory line, but there will still be costs in stretching the 737 line. Get the feeling of the 737 program "polishing the turd"... Not saying the 737 isn't great, but sometmes you gotta take a step back to go forward in the idea of growth. With composites, new engine tech, and new avionics/entertainment to seems things are being left on the table. The 707/727 programs are done with engine configuration. The 737 has a healthy order book. The 717 is done, the 737 is alive and well. The 747 has the -8 for cargo, the 767 is focused on cargo, the 777 is getting a 777x treatment, the 787 is the new long-haul on a small scal of the 777. The 797 isn't here, and as I stated, is the end of the line (sarcasm) but not here yet. The 757 was a huge hit between short haul and long haul. One of the few aircraft labeled a workhorse outside of the dc-9 (md-80/88/90/717) variant.
If Boeing is looking at a 225-250 pax option (737-1000) it seems a reset in thought would be worthwhile. They have stretched a dodge caravan to a hotel van, now a short bus, now a full on school bus.... On the same frame. Just my opinion.
The 737-100 showed up for 85 pax, it's now up to 215 pax in the max. The 757 was designed for 200-275.